In a conventional mobile telecommunication system, like UMTS, the primary station like a base station (or NodeB) and the secondary stations, like the mobile stations communicate together by means of a plurality of channels. Mainly, data channels are used for transmitting data from a primary station to the secondary stations, and control channels are used for giving information on the operation of the communication systems.
Usually, when the primary station wishes to change a transmission parameter for data, like increasing the transmission power of the signals transmitted on the data channels, the primary station applies the same increase in power for the control channel.
However, this leads to an increase of the interference for all the secondary stations, and is not always the most adequate parameter amendment to be carried out.
More specifically, in systems where it is possible for the secondary stations to apply specific beamforming for data transmissions (in UMTS LTE), the range of coverage for the data transmissions increases for the beamforming transmissions compared to other transmissions in normal mode. However, if the control signalling indicating the data transmission parameters (inter alia, the time-frequency resources and MCS (modulation and coding scheme)) is unreliable, the increase in coverage for data will not be advantageous as the secondary station would be unable to receive sufficient information to know how to decode the data (even though if it had that information the SNR would be sufficient for decoding).
One solution would be to apply beamforming to the control channel also. However, in systems such as LTE which have a common control channel containing control signalling for multiple secondary stations, beamforming cannot easily be used for the transmission of the control channel, as the pilot symbols embedded in the control channel are common for all secondary stations—i.e. they are used as a phase reference for decoding the control signalling by all secondary stations, irrespective of whether the data transmissions to a particular secondary station use beamforming or not. Consequently, the pilot symbols in the control channel cannot use beamforming (otherwise they would not provide a correct phase reference for the secondary stations whose control channels do not use beamforming), and therefore if beamforming were applied to the control signalling for some secondary stations, there would be no suitable beamformed phase reference for the control signalling for those secondary stations.